The present disclosure is concerned with determining porosity and/or other properties of porous material with liquid in the pores of the material. Porous material to be examined may be encountered in a number of industries and circumstances. Possibilities include examination of samples of particulate catalysts, chromatography column packing materials, porous pharmaceutical materials as well as many other porous materials which may be in the form of particles or larger objects. A number of physical properties of porous material may be of interest, again depending on industry and circumstances. Some are properties of liquid in the pores, others are characteristics of the porous material, including its porosity which can be expressed as the volume of the pores as a percentage of the bulk volume of the material.
An area of interest to the present applicants is the examination of drill cuttings brought to the surface during the drilling of a subterranean borehole. When drilling a subterranean borehole with a rotary drill, as is done when drilling for oil or gas, it is standard practice to pump drilling fluid from the surface, down the drill string within the borehole to the drill bit and then back to the surface entraining the cuttings made by the drill bit. Drill cuttings are then separated from the drilling fluid at the surface.
There are numerous techniques for investigating the nature and properties of underground strata before, during and after drilling. Examination of drill cuttings is one of this range of techniques.
When drilling for oil or gas, examination of drill cuttings is used in the course of so called mud logging carried out at the well site, that is logging data from drilling fluid returning to the surface to build a geological description of the reservoir, determine hydrocarbon indices, and identify zones from which oil or gas can be produced.
Nuclear magnetic resonance (NMR) has become a recognized tool in the field of petrophysics, both as a laboratory tool and as a downhole well logging tool. The signal amplitude obtained in an NMR measurement is directly proportional to the number of resonant nuclei (usually 1H) in a sample which is being examined, and therefore the signal amplitude is directly proportional to the mass of oil or brine present in a sample.
Although NMR can observe liquids within cuttings, NMR measurements are also sensitive to liquids surrounding the exterior of cuttings, which will include interstitial liquid trapped between the cuttings which are packed together. This liquid at the exterior of cuttings can provide a significant or dominant contribution to the observed signal. Thus it can be impractical or even impossible to use NMR to examine properties of fluid within pores because signals from a resonant nucleus within the pores are inseparable from much stronger signals coming from the same resonant nucleus present in fluid at the exterior of the porous material. Although there have been a few proposals to make use of NMR at a well-site, these have tended to envisage time-consuming cleaning of the cuttings' exterior as a preliminary step which may prove to be impractical to implement at the well site for significant sample volumes. Thus, the majority of studies published on petrophysics from drill cuttings have relied on data generated in laboratories away from the well site.
In summary, obtaining information rapidly at the well site as drilling is in progress is desirable, because the information is then available in near real time as an input to decisions about the drilling. However, there are a number of properties, relevant as petrophysical information, which can be determined through off-site laboratory analysis of drill cuttings but which are rarely if ever determined on cuttings at the well site due to the complexity of the methodology or the size and cost of equipment required. More specifically porosity of the rock from which the cuttings have been drilled is a significant property which is not normally determined at the well site. Others which normally are only available through off-site analysis are permeability, irreducible water saturation and wettability.